Vendor python3 formatargspec, import from collections.abc

Vendor python3 formatargspec, import from collections.abc

Resolved remaining Python 3 deprecation warnings, covering
the use of inspect.formatargspec() with a vendored version
copied from the Python standard library, importing
collections.abc above Python 3.3 when testing against abstract
base classes, fixed one occurrence of log.warn(), as well as a few
invalid escape sequences.

Add DeprecationWarning to the test suite as an error raise
as has been the case within SQLAlchemy for some time now.

Fixes: #507
Co-authored-by: Mike Bayer <mike_mp@zzzcomputing.com>
Change-Id: I121121b3d2dd90e6f3c9b16dec2fc80b9699c400
Pull-request: https://bitbucket.org/zzzeek/alembic/pull-requests/85